The basic chassis of all the DTM cars is made by Dallara. So, it may be a little less than you think. The engines may be spec as well.

I am very much aware of all the new cost-saving rules for this season of DTM, yes including the sharing of the carbon monocoque. The engines are spec, but each manufacture has their own engine program. BMW's P66 engine was new from the ground up.

Starting a brand new program from scratch cost considerable time, money, personal, and resources. Whether the cars share components or not. I wouldn't even want to guess how much time was spent on the body and aerodynamics.